Cooling system

ABSTRACT

The maximum radiating value of a radiator ( 5 ) is not more than the maximum heat value of an IGBT ( 3 ). Even if the radiating capacity of the radiator ( 5 ) is not more than the maximum heat value of the IGBT ( 3 ), the temperature of cooling water returned to the heat absorber ( 4 ) can be maintained low and the size of the radiator ( 5 ) can be decreased because the increasing rate of the temperature of the cooling water returned to the heat absorber ( 4 ) is sufficiently smaller than that of the heating element ( 3 ), due to the heat capacity of the fluid, the heat absorber ( 4 ) and the radiator ( 5 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a cooling system for cooling a heating element, and is effectively applied to a cooling system of an electric circuit such as an IGBT (power transistor) supplying a driving electric current to an electric motor that generates a driving power.

2. Description of the Related Art

Conventionally, in a cooling operation for a heating element such as an IGBT (power transistor), a cooling system comprising a radiator having a radiating capacity larger than the maximum heat value of the heating element, has been used.

Specifically, if the maximum heat value of the heating element is 9 kW, the maximum radiating capacity of the radiator is larger than 9 kW.

Recently, the heat value in an inverter circuit such as an IGBT has increased as the output of an electric motor for driving a vehicle has increased. In response to this increase, it is necessary to increase the size of a radiator.

However, if the size of a radiator is simply increased in accordance with an increase of the heat value of the IGBT, problems with a mounting layout, such as a decrease of a space for mounting a condenser that is a radiator of a vehicle air conditioner and a radiator that radiates an engine (internal combustion engine), or a restriction in the ornamental design of a front portion of a vehicle, occur.

SUMMARY OF THE INVENTION

In view of the above problems, a first object of the present invention is to provide a new cooling system different from a conventional one, and a second object of the present invention is to decrease the size of a radiator which radiates heat absorbed from a heating element.

In order to accomplish the above objects, according to a first aspect of the present invention, there is provided a cooling system for cooling a heating element, comprising a heat absorber (4) which absorbs heat generated by the heating element (3), and transfers the absorbed heat to cooling fluid; a radiator (5) which radiates heat of the fluid, to cool the fluid; and piping means (6) which connects the heat absorber (4) and the radiator (5), to define a circulation passage through which the fluid circulates, wherein the maximum radiating value of the radiator (5) is not more than the maximum heat value of the heating element (3).

Even if the radiating capacity of the radiator (5) is not more than the maximum heat value of the heating element (3), the temperature of the fluid returned to the heat absorber (4) can be maintained low and the size of the radiator (5) can be decreased because the increasing rate of the temperature of the fluid is sufficiently smaller than that of the heating element (3) due to the heat capacity of the fluid, the heat absorber (4) and the radiator (5).

According to a second aspect of the present invention, there is provided a cooling system further comprising a pump (7) which forcedly circulates the fluid between the radiator (5) and the heat absorber (4).

According to a third aspect of the present invention, there is provided a cooling system which is applied to a vehicle having an electric motor (2) which generates a driving power, and which cools an electric circuit (3) supplying a driving electric current to the electric motor (2), comprising a heat absorber (4) which absorbs heat generated by the electric circuit (3), and transfers the absorbed heat to cooling fluid; a radiator (5) which radiates heat of the fluid, to cool the fluid; and piping means (6) which connects the heat absorber (4) and the radiator (5), to define a circulation passage through which the fluid circulates, wherein the maximum radiating value of the radiator (5) is not more than the maximum heat value of the electric circuit (3).

Even if the radiating value of the radiator (5) is not more than the maximum heat value of the electric circuit (3), the temperature of the fluid returned to the heat absorber (4) can be maintained low and the size of the radiator (5) can be decreased because the increasing rate of the temperature of the fluid is sufficiently smaller than that of the electric circuit (3) due to the heat capacity of the fluid, the heat absorber (4) and the radiator (5).

According to a fourth aspect of the present invention, there is provided a cooling system further comprising a pump (7) which forcedly circulates the fluid between the radiator (5) and the heat absorber (4).

Incidentally, the reference numerals in parentheses, to denote the above means, are intended to show the relationship of the specific means which will be described later in an embodiment of the invention.

The present invention may be more fully understood from the description of preferred embodiments of the invention set forth below, together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic view of an embodiment of a cooling system according to the present invention, in a mounted state;

FIG. 2 is a schematic view of a cooling system for an IGBT;

FIG. 3 is a schematic view of a heat absorber;

FIG. 4 is a flowchart of operations of a cooling system according to an embodiment of the present invention;

FIG. 5 is a graph of an experimental result showing a relationship between the time-shift of the heat value of the IGBT 3 and the temperatures of the IGBT 3, the wall surface of the tube 4 a and the cooling water on the side of a cooling water inlet of the heat absorber 4.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the present embodiment, a cooling system according to the present invention is applied to a cooling system for cooling an inverter motor driving circuit comprising an IGBT or the like, which controls the output of an electric motor by supplying a driving electric current to an electric motor for driving a vehicle, mounted on a hybrid vehicle.

FIG. 1 is a schematic view of a cooling system according to the present embodiment in a mounted state. FIG. 2 is a schematic view of a cooling system for an IGBT. FIG. 3 is a schematic view of a heat-absorber.

As shown in FIG. 1, in a hybrid vehicle according to the present embodiment, a heat engine (internal combustion engine) 1 and an electric motor 2 are used as driving sources, the engine 1 and the electric motor 2 are controlled in accordance with a driving state, as follows.

(1) When a vehicle is stopped, i.e., a vehicle speed is substantially 0 km/h, the engine 1 is stopped.

(2) When a vehicle runs, except for decelerating, a driving power generated by the engine 1 is transferred to a driving wheel. When a vehicle decelerates, the engine 1 is stopped, and a regenerative braking in which the kinetic energy of the vehicle is regenerated into the electric energy by the electric motor 2, is carried out.

(3) When a driving load is large, i.e., when a vehicle starts, accelerates, runs uphill, and runs at high speed, a driving power generated by the electric motor 2 in addition to a driving power generated by the engine 1 is transferred to a driving wheel.

In the present embodiment, the driving load is detected based on a vehicle speed and the amount of depression of a gas pedal.

(4) When the remaining amount of electric charge in a battery (not shown) is not more than a charge-starting target value, the power of the engine 1 is transferred to the electric motor 2 and, then, the battery is charged by the electric motor 2 operated as a power generator.

(5) When the amount of remaining electric charge in a battery is not more than a charge-starting target value, in a stopped vehicle, the engine 1 is started and, then, the power of the engine 1 is transferred to the electric motor 2, to generate electric power.

The charge-starting target value refers to a threshold of the amount of remaining electric charge at which the charging operation begins, and is expressed in percentage, assuming that a full charge is 100%.

A radiator 1 a cools an engine cooling water by a heat exchange between air and the engine cooling water. A condenser 1 b cools a refrigerant by a heat exchange between air and a high-pressure refrigerant for a vehicle air conditioner.

As shown in FIG. 2, an IGBT cooling system comprises a heat absorber 4 which absorbs heat generated by a heating element, IGBT 3, and transfers the absorbed heat to a cooling water; a radiator 5 which cools the cooling water by radiating the heat of the water to the atmosphere, based on the heat exchange between air and the cooling water; piping means 6 which connects the heat absorber 4 and the radiator 5, to define a circulation passage through which the cooling water circulates; and an electric pump 7 which forcedly circulates the cooling water between the radiator 5 and the heat absorber 4.

The radiator 5, the condenser 1 b and the radiator 1 a are arranged in this order from the upstream side in a cooling airflow direction, i.e., the front side of a vehicle.

As the heat absorber 4, a microchannel type, in which the heat is absorbed on opposed sides of the IGBT 3, is adopted.

Specifically, as shown in FIG. 3, the heat absorber 4 comprises tubes 4 a which are disposed on opposed sides of the rectangular IGBT 3, and constitute a cooling water passage; and fins 4 b which enhances the heat exchange between the tubes 4 a and the cooling water by dividing the cooling passage in the tubes 4 a, into a plurality of cooling passages.

The IGBT 3 is secured to external surfaces of the tubes 4 a via heat conduction pads 4 c, thermal diffusion plates 4 d, and electric insulating plates 4 e. The heat of the IGBT 3 is transferred to the cooling water via the heat conduction pads 4 c, the thermal diffusion plates 4 d, the electric insulating plates 4 e and the tubes 4 a, in this order.

The heat conduction pad 4 c is made of a paste-like material such as a silicon grease having a high heat conductivity, and restrains the occurrence of air bubbles which may cause a large heat resistance between the IGBT 3 and the heat diffusion plate 4 d.

The heat diffusion plate 4 d improves a heat-absorbing efficiency of the cooling water by widely diffusing the heat locally generated in the IGBT 3, and is of a boiling type or an opposed-oscillating flow type.

The boiling type heat diffusion plate absorbs the heat from the heating element by utilizing a phase change of a refrigerant, i.e., an evaporation because of boiling, and naturally circulates the refrigerant, to diffuse the heat, based on a density difference between a vapor phase refrigerant whose density is reduced due to evaporation and a liquid phase refrigerant whose density is increased due to radiation of the heat to a to-be-heated object such as a heat transfer device body 6 a.

The opposed-oscillating flow type heat diffusion plate diffuses the heat based on the heat exchange between the adjacent passages by oppositely oscillating the fluid in the adjacent passages, i.e., by utilizing a diffusion enhancing effect due to an oscillating flow.

In the present embodiment, as a cooling water, a fluid in which about 50% of antifreeze liquid such as ethylene glycol is mixed with water is adopted.

An electronic control unit 8 controls the number of revolutions of the pump 7, i.e., the amount of cooling water which circulates in a cooling system, and the number of revolutions of an air blower 9 which blows cooling air to the radiator 5, i.e., the amount of the cooling air. The electronic control unit 8 controls the amount of the circulating cooling water and the amount of the cooling air based on the temperature of the IGBT 3.

It is difficult to directly detect the temperature of the IGBT 3. Therefore, in the present embodiment, the temperature of the IGBT 3 is indirectly detected by a method in which the temperature is calculated based on variations of an electric resistance value in accordance with the temperature, and a method in which the wall surface of the tube 4 a, facing the IGBT 3 is detected by a temperature sensor such as a thermistor.

Examples of the control of the amount of the circulating cooling water and the control of the amount of the cooling air, will be described.

In the present embodiment, the monitoring of the temperature of the IGBT 3 is begun when the electric motor is activated, i.e., the IGBT 3 is energized, and the amount of the circulating cooling water and the amount of the cooling air are increased as the temperature of the IGBT 3 is increased so that the temperature of the IGBT 3 is not higher than a predetermined temperature.

If the temperature of the IGBT 3 is higher than a predetermined temperature even when the amount of the circulating cooling water and the amount of the cooling air are maximum, the output of the IGBT 3, i.e., the output of an inverter circuit (motor driving circuit) is reduced.

FIG. 4 is a flowchart of an example of the above operation. The flowchart shown in FIG. 4 will be described below.

The electric motor 2 is activated, i.e., the IGBT 3 is energized and, then, the temperature of the IGBT 3 is detected when the pump 7 and the air blower 9 are stopped (S1, S2).

Whether or not the temperature of the IGBT 3 is not higher than a predetermined temperature is judged (S3). When the temperature of the IGBT 3 is higher than the predetermined temperature, the amount of a circulating cooling water and the amount of a cooling air are increased as the temperature of the IGBT 3 is increased, so that the temperature of the IGBT 3 is not higher than the predetermined temperature (S4).

Specifically, when the temperature of the IGBT 3 is higher than the predetermined amount, at least one of the amount of the circulating cooling water and the amount of the cooling air is increased by a predetermined amount and, then, the temperature of the IGBT 3 is detected again. If the temperature of the IGBT 3 is not higher than the predetermined temperature, the current amounts of the circulating cooling water and the cooling air are maintained, and if the temperature of the IGBT 3 is still higher than the predetermined temperature, at least one of the amounts of the circulating cooling water and the cooling air is increased by a predetermined amount.

When the amounts of the circulating cooling water and the cooling air are maximums, the temperature of the IGBT 3 is detected again, to judge whether or not the temperature of the IGBT 3 is not higher than the predetermined temperature (S5). If the temperature of the IGBT 3 is still higher than the predetermined temperature, the output of the IGBT 3, i.e., the output of the inverter circuit (motor driving circuit) is reduced while the amounts of the circulating cooling water and the cooling air are maintained maximum.

The operation and effect of the present embodiment will be described below.

FIG. 5 is a graph of an experimental result showing a relationship between the time-shift of the heat value of the IGBT 3 and the temperatures of the IGBT 3, the wall surface of the tube 4 a and the cooling water on the cooling water inlet side of the heat absorber 4.

The maximum heat value of the IGBT 3 is about 9 kw, and the heat value in an usual operation is about 3 kw. The heat absorber 4 can absorb a heat of about 9 kw when the cooling water having a temperature of 65° C. is supplied at 12 liter per minute. The radiator 5 can emit a heat of 4 kw when the cooling air having a temperature of 40° C. is supplied at 4 m/s. The piping 6 has a length of 2 m, and the amount of the cooling water sealed in the cooling system is about 0.4 liter.

As is clear from FIG. 5, even when the heat value of the IGBT 3 varies from an usual heat value (3 kw) to a maximum heat value (9 kw), to increase the temperature of the IGBT 3, the temperature increase of the cooling water returned to the heat absorber 4 is not simultaneous with the temperature increase of the IGBT 3 because the cooling water has a large heat value.

Therefore, even if the radiating value of the radiator 5 is not more than the maximum heat value of the IGBT 3, the temperature of the cooling water returned to the heat absorber 4 can be maintained low because the increasing rate of the temperature of the cooling water is sufficiently smaller than that of the IGBT 3.

In the above state, if the heat value of the IGBT 3 is continuously a maximum for a long time, the temperature of the cooling water returned to the heat absorber 4 cannot be maintained low. However, the heat value of the IGBT 3 is seldom continuously a maximum for a long time, and the heat value of the IGBT 3 is usually a maximum for only several seconds (for example, about 2 seconds).

Therefore, even if the maximum radiating value of the radiator 5 is not more than the maximum heat value of the IGBT 3, the IGBT 3 can be sufficiently cooled.

Accordingly, in the present embodiment, the size of the radiator 5 is reduced to be approximately half that of a conventional device, so that the maximum radiating value of the radiator 5 is not more than the maximum heat value of the IGBT 3. Thus, the problems with a mounting layout, such as a decrease of a space for mounting the condenser 1 b and the radiator 1 a, or a restriction of an ornamental design of a front portion of a vehicle, can be eliminated without simply increasing the size of the radiator 5 in accordance with an increase of the heat value of the IGBT 3.

Even if the radiating value of the IGBT 3 is increased, the radiating value and the size of the radiator 5 can be prevented from increasing. Therefore, a cooling air can be supplied to the condenser 1 b and the radiator 1 a that are disposed on the downstream side of a vehicle in an airflow direction with respect to the radiator 5, to enhance the cooling abilities of the condenser 1 b and the radiator 1 a.

Another embodiment will be described below. In the above embodiment, an electric circuit such as the IGBT 3 is described as an heating element, by way of an example. However, the present invention is not limited to this.

In the above embodiment, the present invention is applied to a cooling system of a hybrid vehicle. However, the present invention is not limited to this.

In the above embodiment, a cooling water is forcedly circulated by the pump 7. However, the present invention is not limited to this. The cooling water may be circulated by natural convection, by, for example, a boiling-type cooling device.

The present invention is not limited to the above embodiments provided it is in accordance with the purpose of the invention described in claims.

While the invention has been described by reference to specific embodiments chosen for purposes of illustration, it should be apparent that numerous modifications could be made thereto by those skilled in the art without departing from the basic concept and scope of the invention. 

1. A cooling system for cooling a heating element, comprising a heat absorber (4) which absorbs heat generated by the heating element (3), and transfers the absorbed heat to cooling fluid; a radiator (5) which radiates heat of the fluid, to cool the fluid; and piping means (6) which connects the heat absorber (4) and the radiator (5), to define a circulation passage through which the fluid circulates, wherein the maximum radiating value of the radiator (5) is not more than the maximum heat value of the heating element (3).
 2. A cooling system according to claim 1, further comprising a pump (7) which forcedly circulates the fluid between the radiator (5) and the heat absorber (4).
 3. A cooling system which is applied to a vehicle having an electric motor (2) which generates a driving power, and which cools an electric circuit (3) supplying a driving electric current to the electric motor (2), comprising a heat absorber (4) which absorbs heat generated by the electric circuit (3), and transfers the absorbed heat to cooling fluid; a radiator (5) which radiates heat of the fluid, to cool the fluid; and piping means (6) which connects the heat absorber (4) and the radiator (5), to define a circulation passage through which the fluid circulates, wherein the maximum radiating value of the radiator (5) is not more than the maximum heat value of the electric circuit (3).
 4. A cooling system according to claim 3, further comprising a pump (7) which forcedly circulates the fluid between the radiator (5) and the heat absorber (4). 